By the end of 1993, the Rosetta mission was approved as the third "Cornerstone" Mission in ESA's Horizon 2000 Science Programme.
Its goal is to rendez-vous with and explore Comet Churyumov Gerasimenko.
The Rosetta satellite is flying with the comet during part of its orbit, from 3.5 Astronomical Unit (AU), i.e. 525 million km from the Sun, to 1 AU, i.e. 150 million km. It enables to study the nucleus as well as its gas and dust environment through its approach to the Sun, by remote sensing techniques and by analysis on the ground itself.
The measurements realized will improve our understanding of:
- the internal structure of the nucleus,
- the nucleus nature and the mineralogical, chemical, and isotopical composition of the nucleus, especially its organic component,
- the interaction between the nucleus and the solar wind or the radiation pressure.
After analysis by the scientific community, those data will be used to improve our understanding of cometary nuclei and the comet phenomenon.
The probe was launched on 2 March 2004 by an ARIANE-5 launcher, and made a rendez-vous with the comet in mid-2014. The mission is planned to end on 30 September 2016.
To meet its target, the Rosetta probe will need a trajectory requesting four gravity assists (Earth, Mars, Earth, Earth) allowing to modify its initial trajectory without undue consumption of power and fuel.
This strategy implies however a long-duration cruise (10 years) which will be used to fly-by asteroids. During the remaining time, for reliability reasons as well as operational costs decrease, the probe will be in Cruise mode, with a minimal exchange of information with the Earth. This strategy has already been used with the GIOTTO probe.
When the probe was very far from the Sun in 2012 and 2013, it was put in Hibernation mode, without any communication link with the Earth.
At the beginning of 2014, as the probe was approaching the comet, it woke up to perform several breaking manoeuvres in July 2014 in orbit around Churyumov-Gerasimenko. Thanks to a first global mapping of the surface the most interesting landing sites were localized. In September, the orbit was lowered to achieve a close observation of these sites. On 12 November 2014, the Orbiter carried the Lander at its separation location.
30 minutes later, Philae landed on the comet and started its nominal five-day scientific studies.
It was hoped that the Lander would continue its mission beyond these five days, depending on the power supplied by the solar panels despite the long distance from the Sun.